The invention relates to sensor arrays for detecting substances, and in particular to a chemical sensor that self-corrects measured values for the effects of environmental conditions, such as operating temperature, pressure or humidity.
Resonant sensors use target molecules adsorbed in the sensing material to change properties that are reflected in the resonance frequency. A wide variety of cantilever, membrane and piezoelectric resonator-based sensors have been fabricated using MEMS technology. These sensors generally detect agents through the use of polymer films and coatings with selective adsorption for a specific agent or set of agents. Although these sensors provide a certain degree of sensitivity, it is desirable in many applications to have sensors with even higher sensitivities.
A capacitive micromachined ultrasonic transducer (cMUT) is a micromachined device having a substrate and a membrane supported above the substrate by an insulating material. A variable voltage applied between the substrate and membrane drives the membrane to vibrate and emit sound waves at ultrasonic frequencies. Arrays of cMUTs have been used for transmitting and receiving ultrasonic beam patterns in air and water over a frequency range from 10 kHz to 100 MHz. These cMUTs rely on the very large electric field (E>108 V/m) in the gap of the capacitor to provide an electromechanical coupling coefficient close to unity.
cMUTs are mostly used for medical imaging. In addition, they have been used to indirectly measure various fluid characteristics, based on processing of ultrasonic signals transmitted and received through the fluid. Due to their resonant character, cMUT devices have the potential to be used as sensors, in a manner similar to MEMS cantilever, membrane, and piezoelectric resonator-based sensors.
U.S. Pat. No. 7,305,883 to Khuri-Yakub discloses such arrays of sensors. Sensor elements include a functionalized membrane supported over a substrate by a support frame. The sensor element is connected to an electrical circuit, which is configured to operate the sensor element at or near an open circuit resonance condition. The mechanical resonance frequency of the functionalized membrane is responsive to binding of an agent to the membrane. The exterior surface of each sensor membrane is chemically functionalized to have an affinity for one or more specific, predetermined chemicals. A detector provides a sensor output responsive to the mechanical resonance frequency of the sensor element.
U.S. Pat. No. 8,424,370 to Cable and Steiert discloses a method for analyzing liquid samples by applying a liquid to a cMUT device having an array of sensors, drying the sensors, and electronically detecting an agent bound to each of the plurality of sensors. An electrical circuit provides a sensor output responsive to a mechanical resonance frequency of the sensor. The exterior surface of sensor membrane is chemically functionalized to have an affinity for one or more specific, predetermined chemicals. The mechanical resonance frequency of the sensor is responsive to the adsorption of a substance of interest to the functionalized membrane, and the mass of the substance that is bound to each of the sensors may be determined.
A resonating member of a sensor, such as a functionalized membrane, may generally detect substances of interest through the use of polymer films and coatings with selective adsorption for specific molecules or sets of target molecules. In practice, nearly every resonating sensor is responsive to other physical or chemical parameters, such as temperature, pressure, humidity, light, other interfering chemical species, etc. In real world operating conditions, the measurements of frequency responses (e.g., changes in resonance frequencies of the sensors) may be significantly affected by environmental conditions such as temperature, pressure and/or interfering gases, thereby masking detection or identification of the true mass of a substance of interest on the sensors. A problem to be solved is how to account for these environmental parameters.